Unit 2 electric current dialogue

Demonstrator: When will electrons move?

Student: If given a path, electrons dislodged from the parent atom will move.

Demonstrator: Well, what do you know about the electric current?

Student: The electric current is a quantity of electrons flowing in a circuit per second of time.

Demonstrator: And what is the unit of measure for current? Student: The unit of measure for current is the ampere. One coulomb passing a point in

a circuit per second, the current strength is I ampere. The ampere is therefore a rate unit.

Demonstrator: Why do the electrons move along the circuit?

S tu d e n t: The electrons move along the circuit because the e. m. f. drives them.

Demonstrator: When is the rate of electron flow doubled? Student: It is doubled, if the force is doubled. It means that other factors being constant, the current is directly proportional to the e. m. f. Demonstrator: What other factor determining the magnitude of the current do you know?

Student: This is the ease with which the electrons are allowed to pass along the circuit. This "ease" or conductivity may be defined as the number of amperes per volt in a circuit.

Demonstrator: And when is a current proportional to the conductivity?

S t u d e n t: All other factors being constant, the current is directly proportional to the conductivity. If the conductivity is doubled, the current will be also doubled.

Demonstrator: How is a magnetic field developed? Student: A stream of electrons in a circuit will develop a magnetic field around the conductor along which the electrons are moving. Demonstrator: What does the strength of the field depend upon?

Student: The strength of the magnetic field depends upon the current strength along the conductor.

Demonstrator: And what about the direction of the field? Student: The direction of the field is dependent upon the direction of the current flow.

Demonstrator: When is the current called direct or alternating?

S t u d e n t: If the force causing the electron flow is unidirectional, the current is called direct. The force changing its direction of effort periodically, the current is known as alternating.

Demonstrator: That will do!

EXERCISES I. Translate the following sentences, paying attention to the different meanings of the words in italics:

1. Work is measured by the product of the moving force times the distance through which the force acts in overcoming the resistance. 2. It is best to have the value of an alternating current or voltage vary with time according to the sine wave. 3. The word "phase", when properly used in a. c. terminology, refers to time. 4. The experiment was repeated many times, and the temperature conditions varied slightly. 5. With metal filament lamps the power radiated as light is nearly three times as great as the power radiated as heat. 6. We could study the reaction mentioned above very thoroughly because it lasted over a long rime. 7. Large turbines have an economy of three or four rimes that of the steam units in a small plant. 8. We know that iron molecules are magnets at all times.

2. Translate the following sentences, paying due attention to the meaning of the words in italics:

1. The wire and the electrical source together form an electric circuit. 2. Until recently magnets have been made of hardened steel, molded or rolled, their form being of a great variety. 3. The energy being lost in the condenser as a result of dielectric hysteresis appears in the form of heat being generated within the dielectric. 4. Virtually all substances in liquid or solid state possess the property of electric conductivity to some degree. 5. One can state that there is no sharp distinction between conductors and insulators. 6. The subject of the investigation carried out was of great scientific importance. 7. In all the experiments made the scientist could subject the substance being tested to thermal treatment at different temperatures. 8. If the voltage gradient is made sufficiently high, one can force any atom to let go one electron. 9. The body was acted upon by a force of an opposite direction. 10. The article we shall read next time will deal with the development of radio engineering in our country. 11. Soviet scientists have contributed a great deal to the development of world science and technique.

2. Translate the following sentences, paying attention to the pro­per place of the predicate in the translations:

1. In all calculations of a. c. power, the effective values of current and voltage are used. 2. In testing apparatus in which insulation is used, or in condenser circuits, the maximum voltage is applied. 3. If a resistance is placed in the circuit, the lag of the maximum voltage will be decreased. 4. Whenever energy in any form is released, a force is developed. 5. Mendeleyev left blanks in his table and predicted what the characteristics of the elements, when found, would be.

2. Analyse and translate the following sentences:

1. Connect the carbon and the copper in the solution to the terminals of a Voltaic battery of three or four cells, the carbon being-connected to the negative pole of the battery. 2. At the places where the transformation of energy is to be made, alternating current transformers are placed, the primary coils of these transformers being connected across between the two primary circuits. 3. It is usual to split up the primary coil into two or more coils and to sandwich these in between the secondary coils, or to wind the secondary coil over the primary coil, the object of these arrangements being to reduce the magnetic leakage. 4. The current being created, work has to be spent to maintain it against the resistance of the circuit. 5. Picture the free ions with their electric charges moving rapidly in different directions, one moment being in combination and the next moment free again. 6. The ions being polarized, any attempt to send more current through the cell meets with an opposing electromotive force, due to the tendency of the ions to recombine. 7. Faraday discovered that if we pass the same current through a number of electrolytic cells arranged in series, or one after the other, each containing different electrolytes, the current will liberate in each cell weights of free ions which are chemically equivalent. 8. The cell being charged, a certain quantity of electricity is passed through it. 9. The armature when would represents on the outside a smooth cylindrical surface of cotton covered wires, all placed parallel to the shaft.

2. State the forms and functions of the Participles and translate the following sentences:

1. The region surrounding one or more charged bodies is known as the electrostatic field. 2. The process by which the signal being transmitted is reproduced from the radio-frequency currents present at the receiver is called detection. 3. The electrification produced in a glass rod by stroking it with silk is arbitrarily called positive electrification. 4. The amount of heat deposited depends on the magnitude of the current and the time it flows. 5. Being allowed to come into contact with the rod, the pith ball is repelled by the rod. 6. Having been impregnated, paper can be employed in the manufacture of cables, transformer coils, etc. 7. An electron leaving the surface, the metal becomes positively charged. 8. Practically all metals are conductors of electricity, the conductivity ranging from silver, which is

1. 06 times as effective as copper, to steel, which has only about 0.1 times that of copper. 9. A magnet being broken into two, two complete magnets result, two new poles appearing at the fracture.

2. Learn the dialogue.

3. Supplementary reading. TEXT 1 TYPES OF ELECTRIC CURRENT

An electric current may be produced in a variety of ways, and from a number of different types of apparatus, e.g. an accumulator, a d.c. or an a.c. generator, or a thermionic valve. Whatever the source of origin, the electric current is fundamentally the same in all cases, but the manner in which it varies with time may be very different. This is shown by the graph of the current plotted against time as a baseband a number of examples are illustrated in the figure that is given by the teacher.

1. represents a steady direct current (D.C.) of unvarying magnitude, such as is obtained from an accumulator.

2. represents a D.C. obtained from a d.c. generator, and consists of a steady D.C. superimposed on which is a uniform ripple of relatively high frequency, due to the commutator of the d.c. generator. As the armature rotates the commutator segments come under the brush in rapid succession and produce a ripple in the voltage which is reproduced in the current.

3. represents a pulsating current varying periodically between maximum and minimum limits. It may be produced by adding a D.C. to an A.C. or vice versa. The d.c. component must be the larger if the current is to remain unidirectional. All the first three types of current are unidirectional, i.e. they flow in one direction only.

4. represents a pure alternating current (A.C). The current flows first in one direction and then in the other in a periodic manner, the time of each alternation being constant, in the ideal case the current varies with time according to a sine law, when it is said to be sinusoidal. Considering the time of a complete cycle of current (a positive half-wave plus a negative half-wave) as equal to 360°, the instantaneous values of the current are proportional to the sine of the angle measured from the zero point where the current is about to rise in the positive direction

( де струм повинен почати зростати у позитивному напрямку).

5. represents a type of A.C. with a different wave form. Such an A.C. is said to have a peaked wave form, the term being self explanatory.

6. represents an A.C. with yet another different wave form. Such an A.C. is said to have a flat-topped wave form, the term again being self- explanatory. Both this and the previous example represent cases of A.C. having non-sinusoidal wave forms.

7. represents an example of an oscillating current, and is similar in shape to (d) except that it has a much higher frequency. An oscillating current is usually regarded as one having a frequency determined by the constants of the circuit, whereas an alternating current has a frequency determined by the apparatus supplying the circuit.

8. represents another type of oscillating current which is known as damped. The current again has a constant frequency, but its amplitude is damped, i.e. it dies down, after which it is brought back to its original value.

9. represents yet another type of oscillating current, this time known as a modulated current. The amplitude varies rhythmically between maximum and minimum values. It may even die down to zero.

(j) The next three examples represent various types of transient currents. These transient currents usually die away extremely rapidly, and times (періоди загасання) are generally measured in microseconds. The first example shows a current dying away to zero, and is an example of a unidirectional transient. Theoretically it takes an infinite time to reach abso­lute zero.

(k) represents a simple a.c. transient. The current gradually dies, down to zero as in the previous case, but this time it is an A.C. that is dying away.

(1) represents a peculiar, but not uncommon, type of a.c. transient. The current is initially unidirectional, but it gradually becomes an ordinary A.C. The positive half-waves die away much more rapidly than the negative half-waves grow, so that the final amplitude is very much reduced.

The above examples do not represent all the types of current en­countered, but they serve as illustrations of what may be expected. It will be observed that in all the above cases the current consists of either or both unidirectional and alternating components. In modern electrical engineering alternating currents play a predominant part, so that a knowledge of the a.c. circuit is of basic importance.